The present invention relates to a color image forming apparatus that is useful, for example, as a color printer, a color copying machine, a color facsimile, and the like, and in particular, to a color image forming apparatus capable of forming a color image by overlapping toner images of many colors by using electrophotography.
As a conventional color image forming apparatus, for example, one disclosed in JP 9-304996A is well known.
The following is a description of a conventional color image forming apparatus, with reference to FIGS. 19 and 20. FIG. 19 is a cross-sectional view showing an overall structure of a conventional color image forming apparatus; and FIG. 20 is a time chart showing an image forming operation of the conventional color image forming apparatus.
As shown in FIG. 19, an intermediate transfer belt unit 201 includes an intermediate transfer belt 202, a primary transfer roller 203, a secondary transfer roller 204, a cleaner roller 205, a waste toner reservoir 206, and the like, and toner image of each color can be overlapped on the intermediate transfer belt 202. In the center part of this color image forming apparatus, a group of developing units 208 are provided. Four developing units 207Y, 207M, 207C and 207Bk for yellow, magenta, cyan and black, each unit being of sector shape in cross section, are arranged circularly to form the group of developing units 208. These developing units 207Y, 207M, 207C and 207Bk are installed properly in the color image forming apparatus. The mechanical driving systems and electrical circuit systems are coupled between the developing units 207Y, 207M, 207C and 207Bk and other portions of the color image forming apparatus via mutual coupling members, so that both sides are mechanically and electrically connected. The developing units 207Y, 207M, 207C and 207Bk are supported by a supporting member and collectively rotated by a driving motor, so that they can rotate around a non-rotatable cylindrical drum 209. Furthermore, at the time of image formation, the developing units 207Y, 207M, 207C and 207Bk are moved successively by rotation to an image forming position 210, which opposes the primary transfer roller 203 that supports the intermediate transfer belt 202. The image forming position 210 is also an exposure position for exposing a photosensitive member 218 with a laser signal beam 211.
Inside this color image forming apparatus, a laser exposure device 212 is arranged horizontally below the group of developing units 208. The laser signal beam 211 passes through an optical path 213 provided between the developing unit 207M for magenta and the developing unit 207C for cyan, and through an opening provided in a part of the cylindrical drum 209, and is incident onto a mirror 214, which is arranged inside the cylindrical drum 209 and fixed to the apparatus main body.
In the standing state, usually, the developing unit 207Bk for black is located at the image forming position 210. Therefore, first, the group of developing units 208 is rotated 90xc2x0 so as to move the developing unit 207Y for yellow to the image forming position 210. The laser signal beam 211, which is incident onto the mirror 214, is reflected from the mirror 214 and enters the developing unit 207Y for yellow, which is positioned at the image forming position 210, by way of an exposure window 215 of the developing unit 207Y for yellow. In this case, the laser signal beam 211 passes through an optical path between a developing device 216 and a cleaner 217, arranged on the upper and lower sides in the developing unit 207Y, is incident into an exposure portion on the left side face of the photosensitive member 218, so as to scan the photosensitive member 218 in the direction of the main line. Thereby, the photosensitive member 218 is exposed, so that an electrostatic latent image is formed. Then, the electrostatic latent image is developed by the developing device 216, and thus a toner image is formed on the surface of the photosensitive member 218.
The toner image formed on the photosensitive member 218 is transferred onto the intermediate transfer belt 202. Then, the group of developing units 208 is rotated 90xc2x0, so that the developing unit 207M for magenta moves into the image forming position 210. An operation similar to the above-mentioned formation of the yellow toner image is performed and a magenta toner image is overlapped on the yellow toner image previously formed on the intermediate transfer belt 202. Similar operations as mentioned above are performed using developing units 207C for cyan and 207Bk for black to compose a full color image on the intermediate transfer belt 202. The transfer in which the toner image of each color is transferred onto the intermediate transfer belt 202 is referred to as xe2x80x9cprimary transfer.xe2x80x9d
When the top of the image on the intermediate transfer belt 202 on which the last black toner image is transferred reaches a secondary transfer position, a tertiary transfer roller 219 pushes the recording paper onto the intermediate transfer belt 202. Thus, the secondary transfer of the toner image to the recording paper starts. The toner images of four colors are transferred onto the recording paper together. At the time the secondary transfer starts, the image forming process on the photosensitive member 218 is continued. The primary transfer from the photosensitive member 218 to the intermediate transfer belt 202 and the secondary transfer from the intermediate transfer belt 202 to the recording paper simultaneously proceed.
When the primary transfer of the black toner image from the photosensitive member 218 onto the intermediate transfer belt 202 is completed, the top of the toner images of four colors, which is transferred onto the recording paper, starts to pass through a fixing device 220. Thereby, the recording paper on which toner images of four colors are transferred is fixed and ejected out of the apparatus. Even if the primary transfer of the black toner image from the photosensitive member 218 to the intermediate transfer belt 202 is completed, the secondary transfer from the intermediate transfer belt 202 to the recording paper is continued. Therefore, the photosensitive member 218 and the intermediate transfer belt 202 continue to be rotated. At this time, the photosensitive member 218 is not irradiated with the laser signal beam 211 and the photosensitive member 218 and the developing device are in a so-called idling state.
When the intermediate transfer belt 202 makes 4.5 rotations in total, the secondary transfer of the toner image from the intermediate transfer belt 202 onto the recording paper is completed and the photosensitive member 218 and the intermediate transfer belt 202 stop.
Thereafter, the group of developing units 208 is rotated 90xc2x0, so that the image formation starts sequentially from the yellow again.
By repeating the above-mentioned operations, a plurality of color images are output successively.
Any toner remaining on the intermediate transfer belt 202 is scraped off by the cleaner roller 205 that is in contact with the intermediate transfer belt 202. The scraped-off toner is collected in the waste toner case 206.
In general, in the conventional color image forming apparatus, as shown in FIG. 20, the developing speed and the primary transfer speed were the same in four colors. Also, the developing speed and the primary transfer speed were the same as the secondary transfer speed and the fixing speed.
Therefore, for example, when the time required for the operation of switching the developing unit 207 by rotating 90xc2x0 the carriage 2 and coupling of the photosensitive member 218, etc., is 1.0 second, the developing speed by each toner-of yellow (Y), magenta (M), cyan (C) and black (Bk) and the primary transfer speed is 100 mm/second, and the peripheral length of the intermediate transfer belt 202 is 377 mm, the time for outputting one sheet of color image is expressed by:
Txe2x80x2=1.0xc3x974+(377/100)xc3x974.5=21 seconds.
In order to optimize the cost performance of the apparatus, it is desirable to match each performance (=speed) in development, primary transfer, secondary transfer and fixation. However, there are limitations in making serialization of products or sharing units-parts, and the like, it is rare in fact that each performance is accorded to each other. As a result, for example, although there is a sufficient performance for development and primary transfer, the whole speed of the image formation is determined by the performance in the secondary transfer and fixation.
Furthermore, with respect to the black toner, as compared with toners of other colors, i.e., yellow, magenta, and cyan, there is no limitation in the coloring when the colors are overlapped or the permeability in OHP printing. Therefore, in general, it is relatively easy for the black toner to enhance the developing performance as compared with the color toners. However, in the conventional color image forming apparatus, since the developing speeds of four colors were set to be the same, the characteristics of the black toner are not utilized effectively.
Furthermore, another conventional color image forming apparatus is disclosed in, for example, JP 7-36246A. The structure of this image forming apparatus is similar to that of the above-mentioned apparatus disclosed in JP 9-304996A (see FIG. 19). In this case, the formation of the toner images and the transfer operation onto the intermediate transfer belt 202 are performed in the order of black(Bk)xe2x80x94yellow(Y)xe2x80x94magenta(M)xe2x80x94cyan(C). When a full color image is completed on the intermediate transfer belt 202, a recording paper is conveyed by a second transfer roller 204 and a third transfer roller 219, and at the same time, the full color image is transferred onto the recording paper. The recording paper onto which the full color image is transferred is conveyed to the fixing device 220, and thereby the full color image is fixed to the recording paper, and then the recording paper is ejected out of the apparatus.
When the formation of one sheet of color image is completed, the group of developing units 208 is further rotated 90xc2x0. Then, the developing unit 207Bk for black returns to the image forming position 210, thus completing the preparations for the next color image forming operation.
As mentioned above, in the conventional image forming apparatus having the rotating developing units, when one sheet of color image is formed, the switching operation of the developing unit is performed four times and a predetermined time is required for each switching operation of the developing units. In other words, before the next color starts to be exposed, it is necessary to have switching time including a time for rotating 90xc2x0 the group of developing units, a time required for releasing and coupling the driving coupling of the photosensitive member located at the image forming position and the apparatus main body side, a time when the photosensitive member is driven to be rotated and then the rotation speed becomes stable, a time for charging the rotating photosensitive member to a certain level, and the like.
Such a switching time is not short for an outputting speed per sheet in successively outputting a plurality of color images, thus causing a problem in realizing a high speed color image forming apparatus.
In order to solve this problem, for example, WO98/13732 proposes a color image forming apparatus having a high speed mode as shown in FIG. 21(b). Namely, in the high speed mode, as shown in FIG. 21(b), the order of overlapping the toner images of many colors onto the intermediate transfer belt is not the same every time. The first sheet begins with black (Bk) and ends in cyan (C);, the second sheet begins with cyan (C) and ends in magenta (M); and the third sheet begins with magenta (M) and ends in yellow (Y). In other words, in the normal mode, as shown in FIG. 21(a), the carriage is rotated 90xc2x0 so as to move the developing unit for yellow (Y) to the image forming position every time. However, in the high speed mode, this operation is not performed, the first toner image is transferred by the use of the developing unit of the last color used in the previous image formation.
However, in the color image forming apparatus having the above-mentioned high speed mode, since the order of overlapping the toner images of yellow (Y), magenta (M) and cyan (C) is changed in each recording paper, there is a problem in that colors are misaligned.
It is an object of the present invention to provide an image forming apparatus capable of reducing the total printing time and having an excellent cost performance. Furthermore, it is another object of the present invention to provide an image forming apparatus capable of outputting a plurality of color images successively at high speed and preventing the color misalignment.
In order to achieve the above-mentioned objects, a first configuration of a color image forming apparatus according to the present invention includes a photosensitive member on the surface of which an electrostatic latent image is formed, a plurality of developing units each having a different color of toner and developing means, a developing unit moving means for switching the plurality of developing units by moving them selectively between an image forming position and other positions, a primary transfer means for overlapping sequentially toner images of many colors, which are formed on the photosensitive member by the plurality of developing units, thereby transferring a color image onto an intermediate transfer member, an intermediate transfer member driving means for driving the intermediate transfer member at a predetermined speed, a secondary transfer means for transferring the color image, which is primary transferred by being overlapped on the intermediate transfer member, onto a printing medium, and a fixing means for fixing the color image, which is transferred on the printing medium, to the printing medium, wherein, in several times of primary transfers by toners of a plurality of colors for forming one sheet of color image, the primary transfer by the toner of the last color is performed at a different speed from that of the toners of the other colors.
According to the first configuration of the color image forming apparatus, it is possible to set the performance in the secondary transfer and fixation and the performance in the development and primary transfer separately by differentiating the speed of the developing onto the photosensitive member and the primary transfer onto the intermediate transfer member by the toner of the color other than the last color from the speed of the secondary transfer onto the printing medium and the fixation when the development to the photosensitive member and primary transfer onto the intermediate transfer member by the toner of the last color and the secondary transfer onto the printing medium and fixation are performed simultaneously. As a result, it is possible to optimize the cost performance in the secondary transfer means and fixing means. Therefore, for example, it is not necessary to introduce a large and expensive transfer device and fixing device for the purpose of matching the secondary transfer speed and fixing speed to the high-performance developing speed and primary transfer speed.
Furthermore, in a case where the performance in the secondary transfer and fixation is higher than the performance in the development and primary transfer, by enhancing the performance in the development and primary transfer by black toner and by using the black toner as a last color, it is possible to reduce the total printing time. Since the black toner has no limitation in permeability as compared with the toners of the other colors, there are many choices in base materials and additives and it is relatively easy to enhance the performance in the development and primary transfer.
Furthermore, it is preferable in the first configuration of the color image forming apparatus according to the present invention that the following expression is satisfied:
L1 less than Lp,L2 less than Lp,
wherein L1 denotes a distance between a primary transfer position in which the primary transfer is performed onto the intermediate transfer member by the primary transfer means and a secondary transfer position in which a secondary transfer is performed onto the printing medium by the secondary transfer means, L2 denotes a distance between the secondary transfer position and a fixing position in which fixation is performed on the printing medium by the fixing means, and Lp denotes a length of the printing medium in a conveying direction. According to such a preferable configuration, it is possible to make the whole apparatus compact. In this case, in order to minimize the total color printing time, it is essential to perform the development and primary transfer of the last color simultaneously with the secondary transfer. In the apparatus having such a configuration, by applying the first configuration of the color image forming apparatus according to the present invention, the same effect can be obtained as in the first configuration. Thus, it is possible to utilize the performance of the apparatus most effectively.
Furthermore, it is preferable in the first configuration of the color image forming apparatus according to the present invention that the number of developing units is four, colors of the toner included in the four developing units are yellow, magenta, cyan and black, respectively, and the last color in the primary transfer is yellow or black. According to such a preferable configuration, even if the color misalignment occurs due to the difference in the image forming speed, it is less remarkable than that of the toner of magenta and cyan.
Furthermore, it is preferable in the first configuration of the color image forming apparatus according to the present invention that each of the plurality of developing units is formed integrally as one unit including the photosensitive member. According to such a preferable configuration, since the photosensitive member and toner, both of which are consumables, are integrated into one developing unit, it is possible to minimize the frequency of exchanging consumables, thus to improve the operation efficiently in exchanging consumables.
Furthermore, a second configuration of a color image forming apparatus according to the present invention includes a photosensitive member on the surface of which an electrostatic latent image is formed, a plurality of developing units each having a different color of toner and developing means, a developing unit moving means for switching the plurality of developing units by moving them selectively between an image forming position and other positions, a transfer drum around which a printing medium can be wound and for overlapping sequentially toner images of many colors, which are formed on the photosensitive member, thereby transferring a color image onto the printing medium, and a fixing means for fixing the color image, which is transferred onto the printing medium, to the printing medium, wherein, in several times of transfers by toners of a plurality of colors for forming one sheet of color image, the transfer by the toner of the last color is performed at a different speed from that by the toners of the other colors.
According to the second configuration of the color image forming apparatus, it is also possible to obtain the same effect as in the first configuration of the color image forming apparatus according to the present invention.
Furthermore, it is preferable in the second configuration of the color image forming apparatus according to the present invention that the following expression is satisfied:
L3 less than Lp,
wherein L3 denotes a distance between a transfer position in which the color image is transferred onto the printing medium and a fixing position in which a fixation is performed to the printing medium by the fixing means, and Lp denotes a length of the printing medium in a conveying direction.
Furthermore, it is preferable in the second configuration of the color image forming apparatus according to the present invention that the number of developing units is four, colors of the toner included in the four developing units are yellow, magenta, cyan and black, respectively, and the last color transferred onto the printing medium is yellow or black.
Furthermore, a third configuration of a color image forming apparatus according to the present invention includes a plurality of developing units each having a different color of toner and developing means, a developing unit moving means for switching the plurality of developing units by moving them selectively between an image forming position and other positions, a photosensitive member capable of receiving a color image formed overlapping sequentially toner images of many colors obtained by the plurality of developing units, a transfer means for transferring the color image formed on the photosensitive member onto a printing medium, and a fixing means for fixing the color images, which is transferred onto the printing medium, to the printing medium, wherein, in the several times of development by toners of a plurality of colors for forming one sheet of color image, the development by the toner of the last color is performed at a different speed from that of the toners of the other colors.
According to the third configuration of the color image forming apparatus, it is also possible to obtain the same effect as in the first configuration of the color image forming apparatus according to the present invention.
Furthermore, it is preferable in the third configuration of the color image forming apparatus according to the present invention that the following expression is satisfied:
L4 less than Lp,L5 less than Lp,
wherein L4 denotes a distance between a developing position in which the development is performed onto the photosensitive member by the developing unit and a transfer position in which the transfer is performed onto the printing medium by the transfer means, L5 denotes a distance between the transfer position and a fixing position in which the fixation is performed on the printing medium by the fixing means, and Lp denotes a length of the printing medium in a conveying direction.
Furthermore, it is preferable in the third configuration of the color image forming apparatus according to the present invention that the number of developing units is four, colors of the toner included in the four developing units are yellow, magenta, cyan and black, respectively, and the last color in the development onto the photosensitive member by the developing is yellow or black.
Furthermore, a fourth configuration of a color image forming apparatus according to the present invention includes a photosensitive member on the surface of which an electrostatic latent image is formed, a plurality of developing units each having a different color of toner and developing means, a developing unit moving means for switching the plurality of developing units by moving them selectively between an image forming position and other positions, a transfer means for forming one sheet of color image on a transfer member by bringing the transfer member into contact with the photosensitive member, by sequentially transferring a toner image formed on the photosensitive member, and by overlapping the toner images of each color, in accordance with the switching of the developing units positioned at the image forming position, and a transfer member driving means for driving the transfer member, with at least one of the plurality of developing units having a black toner; wherein the apparatus has a high speed mode for outputting a plurality of color images, wherein when a first sheet of color image is formed, a black toner image is finally overlapped to form one sheet of color image; when a second sheet of color image is formed following the formation of the first sheet of color image, the developing unit moving means is not operated and the image formation continuous with the developing unit for black remaining at the image forming position, thus forming one sheet of color image by first transferring the black toner image onto the transfer member; and when a third sheet of color image is formed following the formation of the second sheet of color image, one sheet of color image is formed in the same order of overlapping the toner images of many colors as that of the first sheet of color image.
According to the fourth configuration of the color image forming apparatus, since a plurality of color images are output successively in high speed mode, it is possible to reduce the printing speed as compared with the case of outputting the plurality of color images successively by overlapping the images of all colors in the same order every time in the normal mode.
Furthermore, in the high speed mode, the order of overlapping the toner images in the first sheet (or third sheet) of color image and the second sheet of color image are different only in that the black toner image is transferred first or overlapped at the end and the order of overlapping the toner images of yellow, magenta and cyan is the same. Therefore, no color misalignment occurs. Moreover, when this configuration is employed, it is possible remarkably to reduce the printing time of the color image forming apparatus having a process in which the transfer member stops every time one color toner image is transferred onto the transfer member and the developing units are switched during a standing state of the transfer member. Moreover, this configuration is also effective in the color image forming apparatus having a process of reducing the driving speed of the transfer member when the developing units are switched. Furthermore, this configuration is also effective in the color image forming apparatus in which the transfer member does not stop temporarily when the developing units are switched and the driving speed of the transfer member is constant, because it is possible to shorten the idle running distance between the first sheet of color image and the second sheet of color image, and thus it is possible to reduce the total printing time when a plurality of sheets are printed. Namely, it is possible to reduce a so-called xe2x80x9cpaper-to-paper distancexe2x80x9d.
Furthermore, it is preferable in the fourth configuration of the color image forming apparatus according to the present invention that the transfer member is an intermediate transfer member for re-transferring one sheet of color image formed by the toner images of many colors onto the recording paper together. According to such a preferable configuration, as compared with the color image forming apparatus without using the intermediate transfer member, that is, the apparatus having a process in which the toner image of each color is overlapped onto the recording paper wound around the transfer drum, it is possible to reduce the inertial property of the transfer member. Therefore, it is possible to shorten the time required for stopping the transfer member between the transfer of toner images of each color and re-operating the transfer member, and to reduce the power required for the transfer member driving means.
Furthermore, it is preferable in the fourth configuration of the color image forming apparatus according to the present invention that each of the plurality of developing units is formed integrally as one unit including the photosensitive member.
Furthermore, it is preferable in the fourth configuration of the color image forming apparatus according to the present invention that the apparatus further includes a normal mode for outputting a plurality of color image, wherein when a first sheet of color image is formed, a black toner image is finally overlapped to form one sheet of color image; and when a second sheet of color image is formed following the formation of the first sheet of color image, one sheet of color image is formed in the same order of overlapping the toner images of many colors as that of the first sheet of color image by switching the developing units.
Furthermore, it is preferable in the fourth configuration of the color image forming apparatus according to the present invention that the apparatus further includes a four-color mode for forming the color image by using four colors of yellow, magenta, cyan and black; and three-color mode for forming the color image by using three colors of yellow, magenta and cyan.
Furthermore, it is preferable in the fourth configuration of the color image forming apparatus according to the present invention that the developing unit for black is positioned at the image forming position during a standing state in which the printing is not performed. Generally, the apparatus takes a standing state with the developing unit located at the image forming position remaining when the printing is completed. According to such a preferable configuration, it is possible to reduce the printing time of the first sheet in printing a monochromatic image, which is used frequently. Furthermore, when the color image is printed, it is possible to start to print usually in the same sequence. Furthermore, the printing time of the first sheet is not changed depending upon the state in which the previous printing is completed.
Furthermore, it is preferable in the fourth configuration of the color image forming apparatus according to the present invention that when the developing units are switched by operating the developing unit moving means for forming the third color image after the second color image is formed, the developing unit for the first color on which the first color image is formed is moved to the image forming position without stopping the developing unit moving means midway. For example, in forming three color images in the order of Y (yellow)xe2x80x94M (magenta)xe2x80x94C (cyan)xe2x80x94Bk (black), Bkxe2x80x94Yxe2x80x94Mxe2x80x94C, and Yxe2x80x94Mxe2x80x94Cxe2x80x94Bk, after the second sheet of color image is formed, by the use of the developing unit moving means, the developing units are switched in the order of Cxe2x80x94Bkxe2x80x94Y. Therefore, it is possible to further reduce the printing time by moving the developing unit for yellow to the image forming position without temporarily stopping the developing unit for black at the image forming position.